There exists e.g. according to FR-A-2771161 in the name of the applicant an installation having at least a web, gas-heated radiant elements arranged according to at least one row stretching out in the transversal direction of the web, substantially over its entire width, and, downstream at least one row of radiant elements, at least a transverse convective system equipped with suction and blowing devices to suck at least part of the combustion products produced by the radiant elements and to blow this part of the combustion products towards the web. The installation generally also has devices to extract the warm gasses resulting from the convective exchanges between the passing web and the combustion products.
The suction and blowing devices have a mixing device, such as e.g. a ventilator, that is, for several known reasons, shifted laterally at the outside of the web, in relation to the median longitudinal axis usually at a large, even extremely large, distance in relation to the width of the web. In that way, the ventilator has to laterally collect the combustion products that are initially divided over the entire width of the web, mix the combustion products and divide them again over the entire width of the web. Such a mixing entails an important consumption of energy.
In addition, such an installation has suction and blowing ducts that, at least in the transverse direction of the web, have an important size. These ducts dissipate thermal energy by radiation and convection. There is amongst other things aspiration of cold air that is cooling down the combustion products. Hence, the temperature of the combustion products blown on the web is considerably lower than the temperature of the combustion products generated by the radiant elements.
Such an installation, although functioning satisfactorily, thus implies a considerable consumption of mechanical energy and also a considerable loss of thermal energy, thus resulting in considerable investment and operating costs, and also occupies a large surface. An already improved system has been described in WO 2005/085729 in the name of the applicant resulting in a reduced consumption of mechanical energy and a reduced loss of thermal energy, lower investment and operation costs, and necessitating less space. This dryer installation is characterized by the fact that the suction and blowing devices of the convective system have at least one suction and blowing device installed opposite of the passing web in relation to corresponding suction and blowing ducts that at least stretch out in the transverse direction of the web, and arranged so as to suck and/or blow the said combustion products in such a way that the vector averages are optimized. The vectors are representing the respective trajectories of the different jets of sucked and/or blown combustion products. This optimization considerably reduces the trajectories of the jets of combustion products and the mechanical mixing energy needed to suck and blow the different jets of combustion products. These shorter trajectories of combustion products require shorter suction and blowing ducts and smaller dimensions corresponding to smaller surfaces that lead to considerably smaller losses of thermal energy by radiation and convection.
Likewise, the temperature difference between the sucked combustion products and the blown combustion products is substantially reduced, thereby increasing the efficiency.
In that way, the thermal transfers between the combustion products and the passing plane can be maximized, and it is also possible to obtain an extremely compact dryer installation in which the combustion products are blown at the highest possible temperature.
Although above described system has already improved the efficiency of the dryer installation to a large extent, there is still a major restriction to the system in that the mixing devices cannot withstand temperatures that are higher than e.g. 350° C., thereby limiting the temperature of the warm blown combustion products.